Help:Video Transcoder, LMH1251, possible ground problem???

[Jerry Lynds]

Hello,

I have built a YPbPr (component video) to RGBs (analog, composite sync)
transcoder using the LMH1251. This also includes the LMH6739 tripple video
buffer.

The probelm is that the screen goes black after just a few seconds, but the
image is stable and clear while it is on. Here is some more diagnosis
info...

When I tried it out for the first time the image had a rolling black bar
that was solid (not waving) and took up half the screen, rolled very slowy.
If I touched a grounded area on the pcb the black bar would increase and get
full of horizontal lines on the top and bottom of that bar, as if the image
was compressed into the center of the screen. Again, this was only when I
touched it. When it was 1st plugged in, for a few seconds, the screen was
fine then the black bar started. If I picked the pcb board up, not touching
ground or anything conductive, and tilted the board, the black bar would
consume the screen and become distorted in a simlar way to when I touched
ground. The amount of distortion would be directly proportional to the angle
and amount of tilt.

Now for the odd part. The next day I pluged it in a went to try it out and
experiment. IT WORKED PERFECTLY! I didn't change a thing. I didn't even
unhook it. I would leave it on for an hour and worked beautifully. I could
touch GND, tilt it and worked 100%. Now, again, it is not working.

This time the screen goes black after about 2 seconds and nothing has
changed. Could I have a bad Capacitor? How much grounding is required? I
basically have one main vein that runs though the pcb and branches off where
needed. When the screen goes black, I need to let it sit for at least 5
seconds before I can turn it on again and get the image.

I am using a LM1881 to get comp sync. Pretty much used the schematic from
the National web site with the exception of the LM1881. The screen is PSOne
lcd (Playstation 1 LCD). Power supply for the transcoder is an old Zip drive
power box 5 Volts 1.0 amp. Power supplu for LCD is from a gutted
playstation. I know this is a really general and vague question, but I hope
this may "sound" like an obvious problem to someone with more experience.
Thanks in advance...


LMH1251 datasheet http://www.national.com/ds/LM/LMH1251.pdf

Here are some pics of the transcoder:

http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0209.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0207.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0221.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0220.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0219.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0215.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0216.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0217.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0213.jpg
http://i241.photobucket.com/albums/ff166/jerrylynds/IMG_0210.jpg

 

[Jan Panteltje]

Hello,

I have built a YPbPr (component video) to RGBs (analog, composite sync)
transcoder using the LMH1251. This also includes the LMH6739 tripple video
buffer.

Sound like a floating CMOS input to me

 

[GPG]

Sound like a floating CMOS input to me

A possible candidate

POWER SAVE MODE
The LMH1251 is equipped with a power saving mode which
is controlled by Pin 21. This pin is a logic level input. The de-
vice will enter a low power mode when the power save pin is
applied with a logic high by the MCU. Under these conditions,
the IC reduces its current consumption to a minimum as spec-
ified in the Electrical Characteristics section. However, the
sync processor and switch will always remain active. During
power save mode, the RGB video outputs are held to the
blank level, while the sync signals are allowed to continue to
be processed and/or passed through. Based on the absence
or presence of sync signals at the output of the LMH1251, the
MCU can determine whether to bring the system to a low
power consumption state.

 

[Jerry Lynds]

POWER SAVE MODE
The LMH1251 is equipped with a power saving mode which
is controlled by Pin 21. This pin is a logic level input. The de-
vice will enter a low power mode when the power save pin is
applied with a logic high by the MCU. Under these conditions,
the IC reduces its current consumption to a minimum as spec-
ified in the Electrical Characteristics section. However, the
sync processor and switch will always remain active. During
power save mode, the RGB video outputs are held to the
blank level, while the sync signals are allowed to continue to
be processed and/or passed through. Based on the absence
or presence of sync signals at the output of the LMH1251, the
MCU can determine whether to bring the system to a low
power consumption state.

I have thought of this, but I left that pin floating so that feature is
disabled. I was wondering if it needed to be grounded though for a logic
low, instead of just left floating...

 

[GPG]

I have thought of this, but I left that pin floating so that feature is
disabled. I was wondering if it needed to be grounded though for a logic
low, instead of just left floating...

It is not disabled unless a logic low is applied.

 

[Jerry Lynds]

It is not disabled unless a logic low is applied.

Got it! Thanks for the help to everyone between the three groups I sent
this to. You were right GPG, I left the "Power Save" pin Floating (thinking
that was ok for a logic low) to disable that feature. I connected the power
save pin to GND via a 1K resistor (Thanks to a schematic by Michael
Schwingen) and now it works great...so far anyway!

Thanks guys,


Jerry Lynds
Halifax, Nova Scotia